As used herein, the term “molten metal” means any metal or combination of metals in liquid form, such as aluminum, copper, iron, zinc, and alloys thereof. The term “gas” means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, Freon, and helium, which may be released into molten metal.
A reverbatory furnace is used to melt metal and retain the molten metal while the metal is in a molten state. The molten metal in the furnace is sometimes called the molten metal bath. Reverbatory furnaces usually include a chamber for retaining a molten metal pump and that chamber is sometimes referred to as the pump well.
Known pumps for pumping molten metal (also called “molten-metal pumps”) include a pump base (also called a “base”, “housing” or “casing”) and a pump chamber (or “chamber” or “molten metal pump chamber”), which is an open area formed within the pump base. Such pumps also include one or more inlets in the pump base, an inlet being an opening to allow molten metal to enter the pump chamber.
A discharge is formed in the pump base and is a channel or conduit that communicates with the molten metal pump chamber, and leads from the pump chamber to the molten metal bath. A tangential discharge is a discharge formed at a tangent to the pump chamber. The discharge may also be axial, in which case the pump is called an axial pump. In an axial pump the pump chamber and discharge may be the essentially the same structure (or different areas of the same structure) since the molten metal entering the chamber is expelled directly through (usually directly above or below) the chamber.
A rotor, also called an impeller, is mounted in the pump chamber and is connected to a drive shaft. The drive shaft is typically a motor shaft coupled to a rotor shaft, wherein the motor shaft has two ends, one end being connected to a motor and the other end being coupled to the rotor shaft. The rotor shaft also has two ends, wherein one end is coupled to the motor shaft and the other end is connected to the rotor. Often, the rotor shaft is comprised of graphite, the motor shaft is comprised of steel, and the two are coupled by a coupling, which is usually comprised of steel.
As the motor turns the drive shaft, the drive shaft turns the rotor and the rotor pushes molten metal out of the pump chamber, through the discharge, which may be an axial or tangential discharge, and into the molten metal bath. Most molten metal pumps are gravity fed, wherein gravity forces molten metal through the inlet and into the pump chamber as the rotor pushes molten metal out of the pump chamber.
Molten metal pump casings and rotors usually, but not necessarily, employ a bearing system comprising ceramic rings wherein there are one or more rings on the rotor that align with rings in the pump chamber such as rings at the inlet (which is usually the opening in the housing at the top of the pump chamber and/or bottom of the pump chamber) when the rotor is placed in the pump chamber. The purpose of the bearing system is to reduce damage to the soft, graphite components, particularly the rotor and pump chamber wall, during pump operation. A known bearing system is described in U.S. Pat. No. 5,203,681 to Cooper, the disclosure of which is incorporated herein by reference. U.S. Pat. Nos. 5,951,243 and 6,093,000, each to Cooper, the disclosures of which are incorporated herein by reference, disclose, respectively, bearings that may be used with molten metal pumps and rigid coupling designs and a monolithic rotor. U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 4,169,584 to Mangalick, and U.S. Pat. No. 6,123,523 to Cooper (the disclosure of the aforementioned patent to Cooper is incorporated herein by reference) also disclose molten metal pump designs. U.S. Pat. No. 6,303,074 to Cooper, which is incorporated herein by reference, discloses a dual-flow rotor, wherein the rotor has at least one surface that pushes molten metal into the pump chamber.
The materials forming the molten metal pump components that contact the molten metal bath should remain relatively stable in the bath. Structural refractory materials, such as graphite or ceramics, that are resistant to disintegration by corrosive attack from the molten metal may be used. As used herein “ceramics” or “ceramic” refers to any oxidized metal (including silicon) or carbon-based material, excluding graphite, capable of being used in the environment of a molten metal bath. “Graphite” means any type of graphite, whether or not chemically treated. Graphite is particularly suitable for being formed into pump components because it is (a) soft and relatively easy to machine, (b) not as brittle as ceramics and less prone to breakage, and (c) less expensive than ceramics.
Three basic types of pumps for pumping molten metal, such as molten aluminum, are utilized: circulation pumps, transfer pumps and gas-release pumps. Circulation pumps are used to circulate the molten metal within a bath, thereby generally equalizing the temperature of the molten metal. Most often, circulation pumps are used in a reverbatory furnace having an external well. The well is usually an extension of a charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of a reverbatory furnace to a different location such as a launder, ladle, or another furnace. Examples of transfer pumps are disclosed in U.S. Pat. No. 6,345,964 B 1 to Cooper, the disclosure of which is incorporated herein by reference, and U.S. Pat. No. 5,203,681.
Gas-release pumps, such as gas-injection pumps, circulate molten metal while releasing a gas into the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals, such as magnesium, from the molten metal. As is known by those skilled in the art, the removing of dissolved gas is known as “degassing” while the removal of magnesium is known as “demagging.” Gas-release pumps may be used for either of these purposes or for any other application for which it is desirable to introduce gas into molten metal. Gas-release pumps generally include a gas-transfer conduit having a first end that is connected to a gas source and a second submerged in the molten metal bath. Gas is introduced into the first end of the gas-transfer conduit and is released from the second end into the molten metal. The gas may be released downstream of the pump chamber into either the pump discharge or a metal-transfer conduit extending from the discharge, or into a stream of molten metal exiting either the discharge or the metal-transfer conduit. Alternatively, gas may be released into the pump chamber or upstream of the pump chamber at a position where it enters the pump chamber. A system for releasing gas into a pump chamber is disclosed in U.S. Pat. No. 6,123,523 to Cooper. Furthermore, gas may be released into a stream of molten metal passing through a discharge or metal-transfer conduit wherein the position of a gas-release opening in the metal-transfer conduit enables pressure from the molten metal stream to assist in drawing gas into the molten metal stream. Such a structure and method is disclosed in U.S. application Ser. No. 10/773,101 entitled “System for Releasing Gas into Molten Metal”, invented by Paul V. Cooper, and filed on Feb. 4, 2004, the disclosure of which is incorporated herein by reference.
Generally, a degasser (also called a rotary degasser) is used to remove gaseous impurities from molten metal. A degasser typically includes (1) an impeller shaft having a first end, a second end and a passage (or conduit) therethrough for transferring gas, (2) an impeller (also called a rotor), and (3) a drive source (which is typically a motor, such as a pneumatic motor) for rotating the impeller shaft and the impeller. The degasser impeller shaft is normally part of a drive shaft that includes the impeller shaft, a motor shaft and a coupling that couples the two shafts together. Gas is introduced into the motor shaft through a rotary union. Thus, the first end of the impeller shaft is connected to the drive source and to a gas source (preferably indirectly via the coupling and motor shaft). The second end of the impeller shaft is connected to the impeller, usually by a threaded connection. The gas is released from the end of the impeller shaft submersed in the molten metal bath, where it escapes under the impeller. Examples of rotary degassers are disclosed in U.S. Pat. No. 4,898,367 entitled “Dispersing Gas Into Molten Metal,” U.S. Pat. No. 5,678,807 entitled “Rotary Degassers,” and U.S. Pat. No. 6,689,310 to Cooper entitled “Molten Metal Degassing Device and Impellers Therefore,” the respective disclosures of which are incorporated herein by reference.
Known coupling-to-impeller shaft connections are usually threaded, and gas can seep past the threaded connections, especially after the threads have been worn after operation of the degasser, causing the graphite threads of the impeller shaft to wear. The leaks waste gas, and if caustic gas such as chlorine is used, the gas can interact with nearby steel causing the steel to oxidize as well as releasing the caustic chlorine gas into the atmosphere creating an environmental hazard.
Another problem with conventional devices is that broken or worn impeller shafts are difficult to remove. The impeller shafts, also called “shafts,” “degasser shafts,” or “degasser impeller shafts,” herein, are usually formed of graphite, silicon carbide or some combination thereof. The impeller shafts are typically connected to a coupling by a threaded connection wherein an internal cavity of a collar of the coupling is threaded and the external surface of the impeller shaft is threaded, and threadingly received in the internal cavity of the coupling. Stress is placed on the impeller shaft as it rotates and the shaft is weakened by the threads, so the impeller shaft tends to eventually break, and it typically breaks just below the coupling and the end still threaded into the coupling must be chiseled out, which is time consuming.
Another known way to couple an impeller shaft to a steel motor drive shaft is by threadingly connecting it to a threaded projection extending from the drive shaft. The projection comprises a threaded outer surface that is received in a threaded bore of the graphite impeller shaft. In this case, the single connection serves to both transfer torque to the impeller shaft and to create a gas-tight seal with a threaded bore in the impeller shaft. The impeller shaft is hollow, having an internal bore through which gas is transferred ultimately into the molten metal bath. Although this design allows for relatively easy removal of the impeller shaft if the shaft breaks, the impeller shaft is not supported or aligned by a coupling and the impeller shaft tends to wobble and the graphite threads in the bore wear quickly. As the fit loosens, the impeller shaft becomes more eccentric in its movement, i.e., it wobbles more, and eventually breaks.
One attempt to solve the problems associated with coupling a graphite shaft to a steel drive shaft is shown in U.S. Pat. No. 5,203,681 to Cooper entitled “Submersible Molten Metal Pump.” This reference discloses a two-piece clamp held in position by a through bolt. Shafts retained by this clamp must include a cross axial bore to allow the bolt to pass through the shaft. This structure would not be used by one skilled in the art to couple a hollow shaft that functions as a gas-transfer conduit because gas could leak from the holes formed as part of the cross axial bore.
Further, many conventional devices do not adequately mix the gas being introduced into the molten metal. The gas can become trapped in a pocket within the impeller or rotor, or is otherwise not properly dispersed into the molten metal. Additionally, if rotated too fast in order to more thoroughly mix the gas and molten metal, “cavitation” can occur. Cavitation is when essentially a whirlpool is created that pulls air from the surface into the molten metal. This causes oxidation at the surface of the bath and slag or other impurities may be formed.